The walls of the part of Copenhagen’s landmark Old Stock Exchange building burnt in a devastating fire have collapsed, a fire brigade spokesman said on Thursday evening. Half of the building burned and the roof was destroyed in the blaze that broke out on Tuesday morning. Now, the walls of that half of the building have collapsed despite efforts to stabilize them, the spokesman said. The fire was brought under control on Tuesday afternoon but the fire brigade was preparing to use containers outside the walls in a bid to stabilize the historic masonry. Before the collapse, the fire brigade had …